Rough Idle/Missing

Its a stock 95 GST. Yhe story. About 3 months ago my son calls up and says all of the lights on the dash are on and won't start. I brought over a new battery and installed it. The alternator was putting out 14.2 volts at the time. The SRS light has neve gone out since. About a month ago the alternator was squeeling really loud and the dash light came on. Replaced the altenator only getting 2. volts. Since then the car has been idleing rough and misfiring. If I disconnect the battery and reconnect, it will run better for awhile and go back to the rough idle/missfire. I don't know how to pull codes on this car and did a search on how to do it with everyone saying to do a search or go to AutoZone. How are the codes pulled? What could be his problem?

The codes are pulled by hooking a code reader up to your ob2(diagnostic)port. Then they will print out what the codes mean. Their is a another way by reading the check engine light when its flashing.

Pulled the codes. Nada. Replaced the plugs from Bosch Plats to Bosch Coppers. No more idle problems.